This is if calls are delayed and would automatically forward/enable voicemail on the Treo 750v.
I would suggest not disabling the voicemail but instead...Try this:
Set the Call Forwarding to Voicemail option to greater than 15 seconds. The first option greater than 15 seconds is 20 seconds
1. From the Today screen tap ‘Startâ€
2. Tap ‘Settings’
3. Tap on ‘Phone’ Icon from ‘Personal’ tab.
4. Tap on ‘Call Forwarding’
5. Select Forward After ‘20 Seconds’ or greater.
6. Tap ‘ok’
-->Hope this helped